Midrasch zu Wajikra 13:7

10) "and he shall wash his clothes": so that he not (render things) tamei (by entering a house). "and he is clean": He does not require letting his hair grow long, and rending his garments, and shaving his hair, and birds. "and he shall wash his clothes": I might think that he is thereby "dismissed." It is, therefore, written (Vayikra 13:7) "and if it shall spread, etc." he is tamei."

13) I might think that the spreading imparts tumah in the beginning, (before the quarantine); it is, therefore, written (Vayikra 13:7) "after he has shown himself to the Cohein." I might think that if the Cohein sees it spreading (the first time he sees him), he should address himself to it (in the context of the laws of spreading); it is, therefore, written (Vayikra 13:7) "for his cleansing" — He does not address himself to it (the spreading) until he sees him (for the purpose of bringing him) from a state of tumah to a state of taharah (i.e., the second time he sees him, as opposed to the first time, when he is not yet in a state of tumah). (Vayikra 13:7) "then he shall show himself a second time to the Cohein.": The Cohein who sees him the first time sees him the second time; and if he died (in the interim), a different Cohein sees him.